1). Field of the Invention
This invention relates to microelectronic and optoelectronic packages and, more specifically, to an underfill material in such packages.
2). Discussion of Related Art
A microelectronic or an optoelectronic package usually includes either a microelectronic semiconductor die or an optoelectronic die mounted to a package substrate. The package substrate provides rigidity to the overall package, and often also provides electronic interconnect between the die and another device. An underfill material is introduced between the die and the package substrate.
The semiconductor die, for example, may have conductive members on a lower surface thereof, and are positioned on top of an upper surface of the package substrate. The conductive members provide electric communication between the semiconductor die and the package substrate. The conductive members also mount the conductor die to the package substrate. The conductive members are, however, relatively fragile. Differences in coefficients of thermal expansion (CTE) between the semiconductor die and the package substrate cause relative shrinkage or expansion between the semiconductor die and the package substrate. The underfill material serves to reduce stresses on the conductive members due to such relative expansion or contraction.
A filler material is usually included in the underfill material. Filler loading is usually required to reduce shrinkage during curing, to control CTE, and to control moisture uptake. Filler loading tremendously increases the viscosity of the film at bonding temperature. The increased viscosity tremendously increases the bonding force per bump and reduces the wetting properties of melted film to all contact surfaces. The increase of bonding force per bump often cracks the chip during chip placement, and poor wetting properties of melted film adversely affect the joint integrity and introduces voids that shorten the joint fatigue life.